This application claims benefit of U.S. Provisional Application Ser. No. 60/017,252, filed May 10, 1996.
The present invention relates to a process for making cation exchange membranes having enhanced properties for use in electrochemical cells.
A variety of known electrochemical cells fall within a category of cells often referred to as solid polymer electrolyte (SPE) cells. An SPE cell typically employs a membrane of a cation exchange polymer which serves as a physical separator between the anode and cathode while also serving as an electrolyte. SPE cells can be operated as electrolytic cells for the production of electrochemical products or they may be operated as fuel cells for the production of electrical energy.
Perfluorinated sulfonic acid polymer membranes are particularly well-suited for use in SPE cells due to excellent chemical resistance, long life, and high conductivity. For such membranes to be conductive in an SPE cell, i.e., to transport cations across the membrane, the membrane must have a certain water content. In cells with aqueous reactants such as the chloralkali process which converts an aqueous sodium chloride solution to chlorine gas and an aqueous sodium hydroxide solution, the needed water is always available due to the membrane being in contact with the aqueous solutions on both sides of the membrane. However, in SPE cells employing gaseous reactants, it is usually necessary for efficient operation to humidify the gases entering the cell, i.e., use external humidification to maintain the water content of the membrane. For example, in a hydrogen/oxygen fuel cell employing a cation exchange membrane, protons are transported across the membrane from the anode to the cathode and, due to movement of protons to the cathode, water in the membrane is also transported away from the anode. Ultimately, if the water is not replenished, the membrane can dry out causing decreased conductivity and thus poor cell performance. Consequently, the hydrogen supplied to the anode (and sometimes also the air or oxygen supplied to the cathode) is typically humidified to prevent the membrane from drying out.
The need for external humidification of an SPE cell is generally considered a significant disadvantage due to the control system and water source (or water recycling system) needed to provide the proper humidity to the membrane. External humidification is particularly disadvantageous for fuel cells designed for use as automotive power sources or for portable electrical power sources due to the increased complexity and weight of a complete system.